Sheet metal remains the primary material utilized in the making of an automobile body, despite the advent of plastics technology. Sheet metal body parts, such as fenders and quarter panels, are formed from metal blanks utilizing conventional stamping presses.
The prior art includes patents directed toward monitoring stamping presses or metal forming operations, such as U.S. Pat. Nos. 3,930,248, issued to Keller, 4,023,044, issued to Miller et al., and 4,750,131, issued to Martinez. The '131 patent discloses a method of detecting faulty parts in a progressive die press having a plurality of forming stations, each of which includes a forming tool and a force transducer. The method includes the steps of producing a preselected number of sample parts while collecting reference force data from the force transducers for a plurality of displacements. The reference force data and the displacement data are applied to a processor which produces a reference curve for each of the forming stations. Production force data are provided to the processor and compared to the curve for each of the displacements and for each of the forming stations. A faulty part is indicated when the production force data for at least one of the displacements is different from the reference force data of the reference curve by a preselected difference.
In any metal stamping process, the thickness of the blank and the work hardening coefficient of the material are two important characteristics that determine stamping quality. The blank thickness is also important for compliance with Federal Motor Vehicle Safety Standards, which specify a certain thickness for automobile bodies. Thus, if the blank thickness falls below a certain level, any parts produced therefrom are unacceptable and become scrap.
Work hardening coefficient is important from the point of view of drawability. Work hardening coefficient can be measured in different ways, such as the limited dome height (LDH) test, which is performed on the incoming material. In spite of all the quality assurances provided by suppliers, the incoming material still needs to be monitored. This usually happens in the form of a spot-check at the stamping plant. Even with the spot-checks, splits in the stamped metal body panels can occasionally occur, due to improper work hardening coefficient.
These splits and cracks not only produce a lot of scrap, they result in production delay. Previously, monitoring the thickness of steel stock manual measurement of the thickness using a tool, such as a micrometer, and measurement of the work hardening coefficient required laboratory instrumentation.
Accordingly, there is a need to be able to measure the blank thickness and the work hardening coefficient of the material in an on-line fashion, while the parts are being produced, so that appropriate action can be taken to prevent the formation of any defective parts or production delays.